1. Field of the Invention
The invention relates to self-crosslinking polyvinyl alcohol compositions in the form of aqueous solutions having an extended stability on storage, to a process for their preparation and to their use for preparing films that crosslink at room temperature. Specifically, the invention relates to methods for preparing sheets and adhesive bonds that are resistant to boiling water.
2. Description of Related Art
Adhesives based on polyvinyl alcohol are known in the prior art. Due to their adhesive force, they can be used in the form of aqueous solutions, and specifically as a raw material for bonding cellulosic substrates such as wood, paper or cards. Examples of such fields of application include the preparation of special paper laminates and spiral or parallel sleeves. Alongside a wide range of other applications, polyvinyl alcohol films also are used as packaging materials, in which context they are used on their own or else as a constituent of laminates in composite films.
A known disadvantage of these applications lies in the hydrophilicity of the polyvinyl alcohol. When polyvinyl alcohol is used as an adhesive film, there typically is poor water resistance of the adhesive bonds or, when it is used in films, it produces an inadequate barrier effect to oxygen under conditions of high gas humidity. By using fully hydrolyzed polyvinyl alcohols which are insoluble in cold water it is possible, for example, for use as a paper adhesive, to bring about a certain level of resistance to the action of cold water. Adhesive bonds resistant to boiling water, however, are not obtainable, nor are water-resistant adhesive films or cast films formed from the easy-to-prepare aqueous solutions of partially hydrolyzed polyvinyl alcohols which are soluble in cold water.
It is known in principle to reduce the sensitivity to water of polyvinyl alcohol by means of a large number of possible reagents, for example, using bifunctional aldehydes (see, Finch, C. A., POLYVINYL ALCOHOL, C. A. Finch (Ed); John Wiley and Sons, New York (1992), Chap. 9).
Polyvinyl alcohol films which are resistant to boiling water and are suitable as a flexible packaging material can be prepared, according to U.S. Pat. No. 4,376,183, from an aqueous composition which comprises in aqueous form dissolved polyvinyl alcohol, dialdehydes capable of crosslinking, and, dispersed in this solution, metal orthophosphates, preferably of aluminum or iron. The resistance to hot water arises after heat treatment.
JP-A 117991/77 (Chemical Abstracts 88:74939) describes the preparation of polyvinyl alcohol molding compounds from aqueous solutions by crosslinking with difunctional aldehydes having at least 3 carbon atoms in the presence of acidic catalysts, including salts, and subsequent heat treatment at 50.degree. C. These polyvinyl alcohol molding compositions are resistant to boiling water.
The resistance to boiling water of wet-spun polyvinyl alcohol fibers can be improved according to JP-A 163609/93 (Chemical Abstracts 119:141112) by treating them with dialdehydes, such as glutaraldehyde, or acetals of dialdehydes, for example tetramethoxypropane, in a medium acidified with sulfuric acid. The aqueous polyvinyl alcohol solution includes boric acid added prior to spinning.
Water-resistant polyvinyl alcohol compositions suitable for adhesives and sizes are described by JP-A 157860/94 (Chemical Abstracts 121:257180). In addition to polyvinyl alcohol, these compositions comprise chitosan, aldehydes, for example glyoxal, reducing agents and free-radical scavengers, for example hydroquinone monomethyl ether. The group of reducing agents includes sulfur compounds, including sodium hydrogen sulfite.
Despite the fact that the prior art provides access in principle to polyvinyl alcohol films that are resistant to boiling water, in the case of crosslinking by way of bifunctional aldehydes having at least 3 carbon atoms in an acidic medium, comparison Example 1 below shows that such compositions have a tendency to undergo rapid gelling, and are thus of only limited stability on storage. This feature is particularly detrimental to their use as an adhesive.